The invention for measuring the radiation returned from a material irradiated by an optical radiation source, which device can be placed in the beam the optical radiation source.
The returned radiation generated when a material is irradiated, for example, by means of a laser, has a frequency which is sufficiently different from that of the laser radiation, to permit its measurement in a very simple manner. In particular, a frequency-selective beam splitter may be inserted into the beam path of the laser, which transmits the radiation of the laser, but reflects the returned radiation (German Pat. Document DE 37 26 466 A1). The returned radiation is coupled out of the beam path, and is then directed to a detector where its intensity is measured. The returned radiation (or the detector signal generated by it) provides information on the treated material, on the respective treatment state and the like.
When high-power radiation sources (high-power lasers) are used, however, it has been found that the transmission capacity of frequency-selective beam splitters is too low, which results in an unacceptable heating of the beam splitter and a weakening of the laser beam. For this reason, a complementary solution was suggested in which the laser radiation itself is reflected on a frequency-selective beam splitter having a reflection layer which is designed to be penetrable by the returned radiation (German Pat. Document 37 39 862 C2). However, this solution has the disadvantage that it changes the direction of the original beam path of the laser, and therefore cannot subsequently be built into or removed from an existing laser treatment device.
It is therefore an object of the present invention to provide a device for measuring the radiation returned by a material treated by an optical radiation, which is suitable for particularly high-power radiation and, if necessary, can also be inserted in existing optical treatment devices, or be removed from them.
The measuring device according to invention is based on the recognition that the reflectivity of frequency-selective beam splitters can be optimized to a greater degree by corresponding dielectric layers, than can their transmission behavior. However, a direct application of this principle conflicts with the demand for a measuring device which does not change the beam path of the laser, and which can be retrofitted and removed. Thus, the above mentioned object of the invention is achieved by means of a combination in which an optical deflecting device deflects the radiation of the optical radiation source out of its original beam path and subsequently guides it back to the same axis, and a frequency-selective element separates the returned radiation from that of the optical radiation source.
In order to achieve a particularly high efficiency, it is advantageous for the optical deflecting device to have elements which reflect only the radiation of the optical radiation source (for example, by means of total reflection). However, it is also possible to deflect the radiation of the optical radiation source through prisms and to provide, at a point of the deflected beam path, the frequency-selective beam splitter which is reflecting for the radiation of the optical radiation source and transmitting for the returned radiation. This should advantageously take place at a site very close to the treated material.